1. Field of the Invention
This invention relates to a method of geophysical prospecting which improves the accuracy of seismic migration. Specifically, the invention uses offset or zero-offset survey measurements to accurately migrate reflectors present in three-dimensional (3-D) surface seismic data, in Vertical Seismic Profiles (VSPs), and in cross-well seismic survey data.
2. Description of the Related Art
In surface seismic exploration, energy imparted into the earth by a seismic source reflects from subsurface geophysical features and is recorded by a multiplicity of receivers. This process is repeated numerous times, using source and receiver configurations which may either form a line (2-D acquisition) or cover an area (3-D acquisition). The data which results is processed to produce an image of the reflector using a procedure known as migration.
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution and coherence due to the long and complicated travel paths between source, reflector, and receiver. In particular, due to the two-way passage of seismic signals through a highly absorptive near surface weathered layer with a low, laterally varying velocity, subsurface images may be of poor quality. To overcome this difficulty, a technique commonly known as Vertical Seismic Profiling (VSP) was developed to image the subsurface in the vicinity of a borehole. In a VSP, a surface seismic source is used and signals were received at a downhole receiver or an array of downhole receivers. This is repeated for different depths of the receiver (or receiver array). In offset VSP, a plurality of spaced apart sources are sequentially activated, enabling imaging of a larger range of distances than is possible with a single source.
The VSP data acquisition may be performed by conveying the receivers downhole on a wireline after drilling of the well has been partially or fully completed. An advantage of the VSP method is that the data quality can be much better than in surface data acquisition. The VSP acquisition may also be done by conveying the receiver array downhole as part of the bottomhole assembly (BHA). This is referred to as VSP while drilling.
U.S. Pat. No. 4,627,036 to Wyatt et. al., the contents of which are fully incorporated herein by reference, gives an early example of the VSP method. Referring now to FIG. 1, there is illustrated a typical VSP configuration for land seismic acquisition. In the exemplary figures, a Vibroseis® source 11 is illustrated as imparting energy into the earth. It is noted that any other suitable seismic source such as explosives could be utilized if desired. In a marine environment, the source could be an airgun or a marine vibrator.
A receiver 12 is shown located at a desired depth in the borehole 14. For the location of the receiver 12, energy would be reflected from the subsurface strata 15 at point 16. The output produced from receiver 12 is recorded by the recording truck 17. In VSP, the receiver 12 would typically be moved to a new location for each shot with the distance between geophone locations being some constant distance such as 50 feet. If desired, an array of receivers spaced apart by some desired distance could be utilized or a plurality of sources spaced apart could be used.
Data obtained by VSP has the appearance of that illustrated in FIG. 2. Wyatt discusses the use of a processing technique called the VSP-CDP method by which VSP data such as those shown in FIG. 2 may be stacked to produce an image of the subsurface of the earth away from the well.
The process of migration of surface seismic data has been used for obtaining images of the subsurface that are better then those obtainable with the CDP or stacking method. In migration, the objective is to position seismic reflections at their proper spatial position: in the surface CDP method, on the other hand, it is assumed that reflections originate from a reflection point midway between the source and the receiver. A commonly used method for migration is the Kirchhoff method in which a velocity model is defined for the subsurface. Traveltimes are computed from the source to a diffraction point and from a diffraction point to the receiver. The actual image of a reflector is obtained by combining data from a plurality of source-receiver pairs to a plurality of imaging points. If the velocity model is reasonably accurate, the signals will interfere constructively at the correct image point. This concept was originally developed for surface seismic data. Wiggins (1984) extended the use of migration to cases where the observation surface is not limited to being a flat horizontal plane. The use of Kirchhoff migration for VSP data has been discussed by Dillon.
The teachings of Dillon are limited to 2-D migration. More recently, VSP Kirchhoff depth migration has been used for 3-D VSP data by Bicquart. As noted by Bicquart, Kirchhoff and other wide angle migration methods are sensitive to velocity error. Velocities are difficult to obtain accurately in surface reflection seismology thus limiting the effectiveness of Kirchhoff migration in structures associated with steep dips. In contrast to surface seismic acquisition, in VSP reasonably accurate velocities can be obtained accurately from the well survey. With good velocity depth information, Kirchhoff depth migration produces a better 3-D depth image in the well vicinity. However, in offset 2-DVSP and 3-DVSP source and receiver are not symmetric with respect to the subsurface imaging points. This asymmetry requires considerable effort in computing weighting factors.
In parallel with the improvements in seismic data processing, particularly migration techniques, there has been continued development of a rather fundamental nature in the kind of data acquired. In recent years, multicomponent seismic data has formed an increasing part of the total amount of seismic data acquired. The reason for this has been the recognition that conventional, single component seismic data is primarily responsive to compressional wave energy in the vertical direction in the subsurface. The conventional data is most commonly acquired with a compressional wave source and hydrophone detectors in a marine environment, or a vertical source and a vertical detector in land seismic acquisition. Additional information indicative of lithology and fluid content of the subsurface is obtainable from knowledge about the propagation of shear waves in the subsurface. Shear wave arrivals are most conveniently detected by receivers with other orientations than vertical. An additional advantage of multicomponent recording is that, even for compressional energy, knowledge of three components of a received signal can provide an indication of the direction from which energy is received at the receiver, and total amount of energy in that direction.
Hokstad has derived equations for prestack multicomponent Kirchhoff migration. The imaging equations are derived with basis in viscoelastic wave theory. The mathematical structure of the multicomponent imaging equation derived by Hokstad allows for computation of separate images for all combinations of local incident and scattered wavemodes (qP-qP, qP-qS1, qS1-qS1, etc.).
A limitation of the teachings of Hokstad is that they do not address the real world problem of 3-D seismic imaging. While the results derived by Hokstad are quite elegant, the examples are limited to 2-D data and do not offer any practical suggestion of dealing with 3-D multicomponent data. The problem of migration of 3-D multicomponent data is addressed in the present invention.